This invention relates to a method of controlling the operating amount of an operation control means for an internal combustion engine, and more particularly to a method of this kind which is adapted to set a desired operating amount for an operation control means, which is optimal to an operating condition of the engine in a predetermined low load region, to thereby achieve smooth operation of the engine.
A method has been proposed, e.g. by Japanese Provisional Patent Publications (Kokai) Nos. 58-88436 and 53-8434, which determines a basic operating amount of operation control means for controlling the operation of the engine, such as a basic fuel injection amount to be supplied to the engine by a fuel supply quantity control system, a basic value of ignition timing to be controlled by an ignition timing control system, and a basic recirculation amount of exhaust gases to be controlled by an exhaust gas recirculation control system, in dependence on absolute pressure in the intake pipe of the engine and engine rotational speed, and corrects the basic operating amount thus determined in response to the temperature of engine cooling water, the temperature of intake air, etc., to thereby set a desired operating amount for the operation control means with accuracy.
However, with the above-mentioned conventional method of determining the desired operating amounts of the operation control means in dependence on the intake pipe absolute pressure and the engine speed (generally called "the speed density method", and hereinafter merely referred to as "the SD method"), the rate of change in intake pipe absolute pressure is small with respect to a change in engine speed when the engine is operating in a low load condition such as an idling condition. This, together with pulsation in intake pipe absolute pressure caused by suction stroke of the engine, makes it difficult to detect intake pipe absolute pressure with accuracy so that an operating amount such as a fuel supply quantity cannot be controlled to values in accordance with operating conditions of the engine with accuracy, often resulting in hunting of the engine rotation.
In view of this disadvantage, a method (hereinafter merely called "the KMe method") has been proposed, e.g. by Japanese Patent Publication (Kokoku) No. 52-6414, which is based upon the recognition that the quantity of intake air passing the throttle valve is not dependent upon pressure PBA in the intake pipe downstream of the throttle valve or pressure of the exhaust gases while the engine is operating in a particular low load condition, e.g. an idling condition, wherein the ratio (PBA/P'A) of intake pipe pressure PBA downstream of the throttle valve to intake pipe pressure PA' upstream of the throttle valve is below a critical pressure ratio (=0.528) at which the intake air forms a sonic fiow, and accordingly the quantity of intake air can be determined solely in dependence on the valve opening of the throttle valve. Therefore, this proposed method detects the valve opening of the throttle valve alone to thereby detect the quantity of intake air with accuracy while the engine is operating in the abovementioned particular low load condition, and then sets the desired operating amounts of the operation control means on the basis of the detected value of the intake air quantity.
However, if, for instance, the manner of setting the fuel injection quantity is promptly switched from the SD method to the KMe method immediately when the engine enters the above particular low load condition from a condition other than the particular low load condition, an abrupt change can occur in the desired operating amounts such as the fuel injection quantity to even cause engine shock and engine stall.
In order to overcome this inconvenience, a method has been proposed by Japanese Provisional Patent Publication (Kokai) No. 60-88830 which determines a desired operating amount of the operation control means by the SD method as well as that by the KMe method, immediately after the engine enters the above particular low load condition from a condition other than the particular low load condition, and continues controlling the operating amount of the operation control means based on the desired operating amount determined by the SD method until the two desired operating amounts determined by the SD method and the KMe method become substantially equal to each other.
However, according to this proposed method the following problem arises when the control method is switched from the SD method to the KMe method: There can occur differences between the actual opening areas of a control valve which bypasses a throttle valve for controlling the amount of supplementary air to the engine, and the throttle valve and the detected opening areas of same, the differences being due to variations in operating characteristics of the sensor for detecting throttle valve opening, installation error of same, clogging of an air cleaner at an inlet of the intake pipe, etc. or possibly due to accumulation of carbon, etc. from the blow-by gases and the atmosphere on the throttle valve and the control valve. Especially, if the supplementary air quantity control valve is formed of a so-called linear solenoid type electromagnetic valve which is adapted to control its opening degree in proportion to driving current, the difference between the detected opening area and the actual opening area will be greater due to the difference between the desired valve opening based on the driving current and the actual valve opening area, i.e. characteristic error of the control valve itself. Because of this error, the desired operating amount determined by the SD method and that determined by the KMe method cannot be substantially equal to each other when the engine enters the particular low load condition, and accordingly the switching of the control method from the SD method to the KMe method cannot be effected smoothly and promptly, rendering the engine operation unstable.